1. Field of the Invention
This invention relates to self-balancing hybrids and in particular to a self-balancing hybrid having a digitally programmable attenuator for varying the impedance of a balancing element.
2. Prior Art
A hybrid is a device for coupling two-wire and four-wire sections of a bi-directional signal path. A hybrid is typically used in telephone voice terminal equipment for interfacing a two-wire bi-directional line used for carrying both transmit and receive voice signals to a four-wire system having a separate unidirectional transmit and receive signal path. It is important that a hybrid isolate the transmit and receive signal paths on the four-wire side to minimize echo perceived at the station remote from the hybrid.
Isolating the transmit signal from the receive signal generally involves matching a balancing impedance with the impedance of the line, which is coupled through a winding of a coupling transformer. The line impedance, however, is not easily determinable since it is mainly due to parasitic capacitance and resistance of the line and is partly dependent on line length, which varies from one connection to another. Prior art systems, therefore, require the balancing impedance to be empirically selected by a technician setting up the terminal equipment. Typically, the balancing impedance consists of a variable resistor in parallel with a plurality of separately switchable capacitors.
In prior art designs, the balancing impedance was manually adjusted. A technician, monitoring trans-hybrid rejection (four-wire transmit and receive signal isolation) closed the appropriate switches and adjusted the variable resistor until the best available trans-hybrid rejection was achieved. This process is time consuming and not wholly accurate since the capacitance is variable only in a limited number of coarse discrete steps through the switching arrangement.
A number of U.S. patents disclose impedance balancing circuits for two-wire to four-wire hybrids. For example, U.S. Pat. No. 4,351,060--Treiber discloses means for automatically digitally synthesizing an output impedance. The output impedance may be a matching line terminating impedance. The device eliminates analog two to four-wire converters by performing the two to four-wire conversion function digitally. Treiber requires a plurality of additional active elements for operation. For example, Treiber requires analog to digital converters, summing means and digital filters.
U.S. Pat. No. 4,357,495--Sweet et al. discloses a balancing system wherein the circuit is balanced by monitoring line voltages. A monitoring circuit is connected to the line voltage supply so that the variations in line conditions cause compensatory variations in the line voltage supply. This approach is undesirable since the parameter which determines line balance, namely the respective impedances, is not manipulated directly. Instead the signal is distorted (the voltage varied) as a means to balance the circuit. Another example of a two-wire to four-wire hybrid having an adjustable impedance system is disclosed in U.S. Pat. No. 4,365,119--Chung et al. Chung et al discloses an automatically adjustable impedance system for a two to four-wire hybrid which uses a programmable control unit to automatically adjust an impedance circuit and an adjustable capacitor. Chung further discloses an active element for supplying a signal to be cancelled to the inverting input of a difference amplifier coupled to a transformer.
U.S. Pat. No. 4,918,725--Takahashi discloses a terminal impedance setting circuit for a two-wire to four-wire converting circuit. The circuit includes a semi-conductor switch for selecting any of a plurality of shunt resistors in response to a command for setting a terminal impedance value. Takahashi also uses negative feedback circuits and amplifiers.
The art further contains a number of circuits for providing balancing impedances, not specifically for two-wire to four-wire hybrids. Included is U.S. Pat. No. 3,177,295--Lane et al. Lane et al. discloses a self-regulating two-terminal impedance network for use in the telephone system substation. The circuit is an automatic regulating arrangement for effecting automatic sensitivity regulation in impedance, dependent upon the magnitude of direct current flowing in the line wires and supplied from an exchange.
U.S. Pat. No. 3,548,111--Kleissl discloses means for terminating the remote end of a subscriber line with a coupling circuit whose impedance matches the line impedance. The invention provides for coupling a partly capacitive supplemental impedance means at the remote end in parallel across the subscriber lines.
U.S. Pat. No. 4,004,102--Ott discloses an automatic impedance matching network for transmission lines including an impedance monitor which provides control signals to a low frequency and a high frequency compensator. The disclosure of Ott alters the impedance of the transmission line in response to the frequency of the signal being transmitted.
U.S. Pat. No. 4,346,267--Dijkmans discloses a hybrid circuit for coupling a four-wire transmission path to a two-wire path including a balancing impedance. The balancing impedance is not automatically adjustable.
Martin, U.S. Pat. Nos. 4,567,331 and 4,622,442, disclose two-wire to four-wire coupling hybrid circuits having unspecified balancing networks known as BALNET.
U.S. Pat. No. 4,742,540--Schingh discloses an electronic switching network for switching one or two impedance balancing networks through a telephone line under microprocessor control.
U.S. Pat. No. 4,791,668--Pringle discloses a line interface circuit having a predetermined source or terminating AC impedance selectable by means of semi-conductor switch elements.
U.S. Pat. No. 4,881,262--Meschkat et al. discloses an electronic hybrid circuit having a predetermined complex internal impedance for providing circuit balance.
A circuit is needed which provides a necessary balancing impedance for a two-wire to four-wire hybrid to eliminate echo. A simple system is required, one that does not need a plurality of additional active devices such as summing amplifiers, digital filters and nulling signal producing amplifiers. Preferably the system should vary the balancing impedance without altering the signal levels.